MATEC Web of Conferences 77, 06003 (2016) DOI: 10.1051/matecconf/20167706003 ICMMR 2016 Study on Wellhead Growth in Deep Well Gao Baokui, Ren Jingweiand Gao Liang Department of Petroleum Engineering, China University of Petroleum Beijing, Beijing, 102249, China Abstract. Wellhead growth has occurred in many deep wells in China recently. In the paper, casing axial force and wellhead movement are calculated and compared in regard to moments that free sections of surface casing appear. The parameters considered include preliminary casing tension, free section length and change in wellbore temperature. The moment that surface casing free section comes into being has great influence on wellhead movement, so it is discussed in detail. According to comprehensive investigation on deep well and ultra-deep well accidents, wellhead growth is based on the fact that before production the cement sheath of surface casing has already failed, and during production wellhead grows up due to wellbore temperature increases. The most possible reason of surface casing cement sheath failure is bad cement job quality. Bad cement sheath cannot support casing axial loads and separates from casing. The free section of surface casing will experience large axial force cycle which is harmful to wellbore integrity. Effective measures to remedy wellhead growth is supporting the wellhead with a large base set on ground, rather than pads fixed on conductor. 1 Introduction 2 Models of wellhead growth Wellhead growth is usually occurred in thermal wells and offshore self standing platform wells. In a normal thermal 2.1 Wellbore structure of deep wells well, only production casing is connected with wellhead A typical wellbore sketch of deep well in China is drawn and in most cases the well is cemented to surface. When in Fig. 1. injecting steam or hot water, wellbore is heated, cement sheath may separate from casing and wellhead growth will take place. After injection being stopped the casing will shrink and wellhead will return to its original position, even subside[1, 2]. In an offshore oil well, surface casing has unavoidable free section from mud line to wellhead, therefore some factors, such as casing temperature and annulus pressure, can induce wellhead movement during subsequent constructions. For high level safety, study and achievements are mainly in offshore wells, but calculation methods are generally alike[3-5]. There are little open reports about wellhead growth or subsidence in onshore deep wells, and there is no mechanism study. In fact, accidents of wellhead growth or subsidence in deep wells do exist[6, 7]. In an abnormal deep well all casing strings are top cemented, so the wellhead growth mechanism is more complex[8- 10]. As an important component of wellbore integrity, wellhead growth in deep well must be solved. With regard to deep well construction procedure, Figure 1. Typical wellbore sketch of deep well. change in wellhead position and casing axial force is calculated in this paper. Parameters as casing pretension, Generally, wellbore structure has the following free casing length and thermal effect are considered. The characters: moment and after effect that surface casing section - The first pipe is conductor with depth ranged from becomes free are investigated in detail. 100m to 300m, cemented to surface and will not connect with wellhead. © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). MATEC Web of Conferences 77, 06003 (2016) DOI: 10.1051/matecconf/20167706003 ICMMR 2016 - Surface casing is cemented to surface, and it should afterward. If casing has scheduled free section, casing directly support wellhead loads. will have suspended load when setting at wellhead, and - Based on drilling conditions, intermediate casing and the load is controllable, which is the axial force at top of production casing may be top cemented or not top free casing section. cemented (has free section). - The liner, if there is, doesn’t participate in wellhead growth. 2.3.2 Formulas for calculation wellhead growth - The main wellbore consists of surface casing, If wellbore temperature changes, free casing section will intermediate casing and production casing; these casings have thermal effect, and the formula is joint together at wellhead. d(δl)=αΔTΔL (1) 2.2 Mechanism of wellhead growth -1 where α is linear expansion coefficient of casing,ć ; ΔL ΔT Treat wellhead as a rigid body, then three layers of is micro-segment of free casing section, m; is change casings would grow up simultaneously at top ends. The in temperature of casing micro-segment, ć ; d(δl) is stiffness of surface casing is much larger than others, so elongation of casing micro-segment due to thermal effect, an obvious wellhead growth means that surface casing m. must have a considerable section that can move axially. The temperature difference is different with depth, so Other casings can also have such intervals, and they are integral is needed to obtain total heating effect. termed as free sections throughout this paper. Normally, Both temperature effect and wellhead growth of free free section of surface casing comes from bad cement job casing section will associate with the relationship quality. between casing axial force and deformation, which can The principal work to be done is to find the moment be written as that surface casing free section appears. But wellhead growth is generally observed during production, so the dF=(EA/L)dL (2) task is very difficult. In order to solve this problem, five E A moments are proposed as follows: where is Young’s elastic modulus, kPa; is cross 2 L • Case-1, before hanging intermediate casing, sectional area of casing wall, m ; is free casing length, dL dF • Case-2, when hanging intermediate casing, m; is axial deformation of free casing section, m; is • Case-3, when hanging production casing, change in axial force of free casing section, kN. • Case-4, when hanging tubing, Take wellhead as rigid body, the resultant force of • Case-5, during production. wellhead at vertical direction is zero, the equation can be It is impossible to give precise conditions of cement expressed as sheath failure. So in the following investigation, primary F +F ++F = work is performed under the assumption that surface 1 2 n 0 (3) casing free section appears outright while in different where n is the number of forces acting on wellhead; Fi moments. Casing axial force and wellhead growth are (i=1,2,,n) is ith axial force, kN. main parameters to be calculated. More formulas can be found in reference 1. In engineering, it is hard to predict the moment when surface casing free section appears. The most optimistic expectation is to monitor it timely. In view of study and 2.3.3 Temperature field prevention, any possibility should be considered to find out the causality. Temperature increment is the motivator of wellhead Wellhead growth reported now is a height change growth. So it is necessary to calculate change in compared with previous position. If free section forming temperature in every free casing section. But the moment is not identified, it is impossible to trace calculation is very complicated, which involves wellbore wellhead movement before the so called growth. In this structure, flow rate, fluid components, formation original paper, the reference position of wellhead is the height of temperature, flow period, and so on. In the following surface casing top just after it is cemented. This position calculation some results are used directly without doesn’t distinguish whether surface casing has free analysis in detail. section. 2.3.4 Simplification and hypothesis 2.3 Calculation method In following calculations, some simplifications and hypotheses are used as follows: 2.3.1 Initial axial force at top end of casing • Wellbore is vertical, • The influence of conductor is neglected, Every casing will be connected to wellhead after • The influence of installation and remove of blowout cementing. If top of cement reaches to surface and preventer is neglected, cement job quality is good, there will be solidified • The length of every free section is given, and its pretension in casing wall. It can be regarded as tension of potential extension is neglected. casing to wellhead if this casing appears free section 2 MATEC Web of Conferences 77, 06003 (2016) DOI: 10.1051/matecconf/20167706003 ICMMR 2016 3 Case study spring coefficient (k), is obtained and listed in the fourth column in Table 2. In this section a deep well in west China is taken as The well depth is 6600m, in situ temperature is about example, casing axial force and wellhead growth are 190ć, wellhead temperature is about 90ć when gas calculated and compared. The length of casing free flow rates is 3.0×105m3/d. Based on comprehensive section is just a possible value in the field, namely is not analysis of flow rates, components, wellbore structure, necessarily the real value because real result is impossible flow time and free section length, free elongations (ΔLT) to be identified. from thermal effect of each casing are listed in the fifth column in Table 2. 3.1 Basic parameters Table 2. Casing parameters for wellhead growth calculation. This deep well is top cemented for every layer of casing, Casing F0 (kN) L0 (m) k (m/10 kN) ΔLT (m) and the wellbore structure is shown in Figure 1, detailed 13-3/8" 2416.3 170 0.000661 0.0306 parameters are listed in Table 1. 9-5/8" 762.57 170 0.000977 0.0714 Table 1. Detailed parameters of wellbore structure. 7" 1313.7 500 0.007970 0.6480 Hole Casing outer Casing wall Casing diameter diameter thickness Tubing initial suspending force is 200kN, and ground (mm) (mm) (mm) equipment is simplified as 100kN vertical load.